Gravity bike

This bicycle has no pedals and really nothing that resembles a seat. It’s not so much a way to get around as it’s a way to get down. Down from the mountain, and down low to the pavement. This is a gravity bike built by the guys at S.I.N. Cycles.

The frame is a long triangle which stretches the wheel base to make room for the rider. After getting the layout just right and welding the frame together they added the rather unorthodox building step of pouring molten lead into the hollow frame tube. This bike is used to descend a mountain road with the force of gravity, so the extra weight will help pull it just a bit faster. There’s one thing you’ll want to make sure is working before you climb aboard — the single front brake which you’ll need when coming into the curves. Check out some ride footage on this thing in the video after the break.

Traction on two wheels is vastly different. Weight is almost not a factor, that’s why extremely lght weight motorcycles tend to make it around a track faster than ones with the same geometry, gearing, and power to weight ratios, but more weight.

And (@Wilson) you actually have three competing forces there, the friction of the tires on the pavement too.

A lead block slides down the roof of a house and puts a crater in your driveway much faster than an empty cardboard box of the same size.

Can someone explain to me why anyone would care to build one of these? Why not ride a professional air-dynamically designed bike down? I see the similarities between this and the long board but long boarding can be done without having to go down hill while this cannot be ‘kicked’ for long distances like the long board.

“Well, it’s a skycycle actually, honey. It’s not a bike. A skycycle. There are only two skycycles in the whole world and i have them both. I had three, but the third one is in the bottom of the Snake River Canyon.”

At least you get to go 200mph, right? That’s a very unique experience on a bike, and at least it’s a massive reward to offset the massive risk.

Here I can’t see any reward here to make up for the compromised position and bike weight. It’s good they’re at least wearing head protection and leather to protect against road rash. But on a motorcycle, I’ve been told if you know you’re going to wipe out, you’re supposed to launch yourself clear of the bike so it can’t land on or drag you; and I can’t see that being easy in this rider position.

Why build it? I’d ask “why not?” Maybe build it because he can? Perhaps just to experiment with a concept or to see it run in a comparison?

As practicality goes, Why jump from a helium balloon 23 miles high? Why race a stock car at 170mph when the chances are good you’ll burn it up or crash? Why build many things here on HaD? (No offense, guys.)

Actually, adding weight doesn’t make you go faster. Sure, it increases the force pushing you down the hill, but it increases your inertia by the same amount. You’ll see the same acceleration either way. Having more weight *will* help you maintain speed when the road levels out.

I don’t get the added lead in the frame – mass cancels out when calculating downhill acceleration, so the bike would be no faster.

It could have some advantages beyond speed, though. On flat land after a down hill, it might allow for a greater distance before slowing due to momentum, but that assumes the extra weight does not add other inefficiencies, such as bearing friction. It would not allow any greater distance uphill for the same cancelation as the downhill speed. Also, it might make cornering better, since it lowers the center of gravity a bit, but that would have to be tested to see if it adds a measurable benefit.

Personally, I would think being able to easily carry it up hill would outweigh any benefits of a lead-filled frame, but either way this looks insane and fun and I want to try it!

LOL, I thought I was going to see more comments like this one. This thing seems so funny to ride on!

But come on, “the extra weight will help pull it just a bit faster”, WTF? The extra weight can help to improve the equilibrium/balance/center of gravity and increase the momentum of the whole thing.

Another stupid mistake: front brakes! Protip: Go to the highest hill you can find, go down with your bike and try to use the front brakes when you’re at terminal speed. Just try. Your gravestone will have these words: “He tried to use the front brake (ROFLMAO) R.I.P.”.

Go to the highest hill you can find, go down with your bike and try to use the front brakes when you’re at terminal speed.

I assume that instead of “use” you mean “overapply.”

Similarly, you’ll find yourself in a world of fail if you overapply the rear brake in your death-scenario.

All things in moderation. Brakes are not (usually) binary.

Front brakes are always useful because that is where the most traction is under braking, no matter the incline or speed, on any vehicle with two axles that is traveling forward.

I own a bike which is easily capable of throwing me over the handlebars at perfectly casual speeds on flat ground, if the front brakes are handled roughly. It isn’t much different on a hill. And using too much rear brake results in similarly-catastrophic happenings, though by totally different mechanisms.

If I had to choose between having a bike with a brake on the front wheel or a brake on the rear wheel, I would (without hesitation) choose to have a front brake; no matter what.

Single brake on the front? Umm, that seems a bit dangerous to me, pull it hard enough and the back will flip up and over thereby causing you to hit the pavement hard! I’ve done it on a bike, nearly killed me.

Front brake allows much greater braking forces, a rear brake could very easily lock up the rear wheel, however on the front it causes the weight to load up on the front tire, increasing the contact patch slightly and allows you to fully utilize all available braking force.

Anyone who has any kind of control and experience will know just how hard they can squeeze before doing an endo. 100% of the time, a front brake is more effective.

On the street I don’t even bother with my rear brake on my motorcycle unless I need to stop in an emergency.

Even on flat ground, the ratio of braking power for the front and back brakes is usually about 80/20. Going fast and on a downhill, the rear brake becomes even more useless. Mountain bikers don’t use the rear brake for this reason as all it does is lock up the back tire and tear up the trail – considered a major no-no.

If you’re flipping over the handlebars, you need to learn your brake modulation and weight distribution on the bike better. Just mashing down on the brake levers is about as useful for stopping quickly as jamming the brake pedal down full force on a pre-ABS car.

MTB riders most certainly do use the rear brake.. because when the front wheel stops turning you eat sht. dirt bikes.. grab the front in loose terrain, it locks up and you eat sht. lock the rear up and its called steering.

If you gently apply the front brake (for control or slight slowing) while having much of your body and/or bike mass in the rear, or by simultaneously shifting your weight to the rear, you don’t have to worry that much about going ass-over. It’s the ones who start to stand or lean forward while applying a HARD front brake that more often than not end up kissing the asphalt.

You have two competing forces, weight and air resistance. Since the air resistance is a function of only the cross-section and not weight, the more weight, the more effective force pulling the bike down the hill.

Actually, three competing forces – you are forgetting gravitational time dilation being increased by the all that lead. Time will go slower for the rider because of the lead, which
defeats the purpose of adding it in the first place :-)

Didn’t any of you ever do pinewood derby as kids? The heavier car always wins, that’s why there’s a weight limit. It’s down to how much force you are able to bring into play to counteract drag. F=MA. Heavier car, more force, overcomes more drag, goes faster. Are we all assuming that dropping a feather and a bowling ball off your roof is going to result in a tie?

In addition to the previously mentioned tire friction and air drag that the extra mass helps to overcome, I would think that the additional mass could also help in overcoming the rotational inertia of the tires/wheels. … Assuming you don’t add mass to your tires/wheels (here the mass was added to the frame).

There was actually a physicist once who made a bike that used gravity to move even on slight inclines. Call this BS though cause I don’t remember his name except he was japanese.

It’s one of those really big things that get eclipsed by really sloppy things in applied sciences.. It can actually be amplified, and coefficient lowered compared to the design I seen(weight+aerodynamics)

2 huge mistakes i see here:
– using the front brake when turning (every motorcycle ricder will tell you that you shouldn’t, or in the worst case use the rear brake)
– not moving your body up when taking the turn in order to shift your gravity center, therefore allowing you to corner faster

yeah, no center of gravity shift which is what will steer you above 15mph, also when the guy crashes he puts his outside leg out causing the bike to run wider. if he’d dragged the opposite leg he might have made it.

Gravity Bikes work the same as a Soap Box Derby Car in the long run. I don’t care what you think your physics classes told you, heavier (within breakdown of the bearings) gives you more speed.

Our family races Soap Box in both the NDR and All American and I can tell you that weight makes a HUGE difference in speed. A novice driver in a Stock car at 120lbs will run out (on our hill in KC) at about 20MPH where a Stock driver at 200lbs will run out at 31MPH. Same exact car except for the weight, and in our test cases we also made sure that the windage from the driver was near the same by cheating and using the same driver with more weight using the same CG.

That being said, you also have to take into account the inclination of your hill and the CG of the car when you setup the weights inside of it. Some times its better to have lower weight stacks to lower your CG other times you want one weight stack (front, rear, center) to be high to raise the CG on the car thus giving you an advantage either on push down slope or pull up incline.

Just in case anyone is in the Mid-West and wants to come out the first Annual No-Gas Nationals is this weekend too:

They got their Physics wrong. Bearing friction is negligible. Rolling resistance is fairly low and is fairly constant up below 60 mph. Terminal velocity is reached when the force in the direction of motion (proportional to the weight) is canceled by the force of the oncoming wind(proportional to the square of the velocity). More weight will definitely buy you more speed